1. Field of the Invention
This invention relates to a process and apparatus wherein fluidized solids are used in the treatment and conversion of hydrocarbons. It is applicable to systems wherein such fluidized solids are continuously recycled between a reaction zone and a separate regeneration or reactivation zone, especially where the atmosphere in the reaction zone is different from that in the regeneration zone and no mixing of the atmospheres of the two zones can be allowed to take place.
2. Description of the Prior Art
Extensive commercial utilization of such fluidized systems has been made in the petroleum industry for the catalytic cracking of hydrocarbons. As shown in Hydrocarbon Processing, September 1972, pages 131 to 138, catalytic cracking processes are well known to those skilled in the art. For example, the cracking reactions may take place at a temperature in the range of about 800.degree. to about 1100.degree. F., preferably from about 900.degree. to 1100.degree. F. Pressures employed in such cracking processes may range from about atmospheric to about 50 pounds per square inch gauge (psig) with a preferred range of pressures from about 20 to about 35 psig.
It is known to circulate finely divided solids between a reaction vessel and a regeneration vessel without the use of slide valves as shown, for example, in Packie's U.S. Pat. No. 2,589,124 in which solids are circulated between two vessels into U-shaped tubes, each of which has a riser section into which a controlled amount of gas is injected to regulate the rate of flow of the solids. Utilization of the Packie type U-bend with control risers sets certain limits on the relative arrangement of the two vessels. It is not always economical to arrange the vessels such as would be required for an efficient U-bend system of conveyance of solids. Furthermore, in such systems, in which the regenerated catalyst withdrawal standpipe is eccentrically arranged in the regeneration vessel, expandable seals are required between the grid which supports the fluidized bed present in the regeneration vessel and the vessel wall to absorb the thermal expansion of the grid, or between the standpipe and grid. These seals are subject to failure which results in loss of catalyst and, eventually, in costly shut-down of equipment. This problem is particularly aggravated in high temperature regeneration procedures.
The conventional conduits for catalyst circulation from one vessel to another have been vertical standpipes, sloped standpipes, U-bends and sloped risers. Recent unit configurations in catalytic cracking have generally utilized either sloped standpipes and vertical risers or U-bends. The difficulty with both of these schemes, but particularly the U-bend, is that aeration air can be released in the sloping or horizontal section of the conduit and form a bubble which in turn interferes with catalyst flow. This condition does not generally occur with fine catalyst containing 10 percent of 0 to 40 micron size fines; however, the condition becomes troublesome when these fines drop to 2 to 3 percent, and the 80 plus micron size particles are greater than 30 to 35 percent.
It is also known to crack hydrocarbons in a vertical transferline reaction zone which extends straight into a cyclone-stripper vessel as shown in Hydrocarbon Processing, September 1972, page 133.
It has now been found that improved results can be obtained with a catalytic conversion process and apparatus having a specified combination of features.
Furthermore, it has been found that solids can be circulated between the two vessels in a conveyance system utilizing only a single variable orifice control valve and without the necessity for a control riser section to convey the solids to the regeneration vessel. Moreover, in the preferred embodiment of the apparatus of the invention, expandable grid seals can be omitted from the regeneration vessel and from the reaction vessel.